Reducing The Spectral Bandwidth Of Lasers

Abstract

A laser system for semiconductor inspection includes a fiber-based fundamental light source for generating fundamental light that is then converted / mixed by a frequency conversion module to generate UV-DUV laser light. The fundamental light source includes a nonlinear chirp element (e.g., a Bragg grating or an electro-optic modulator) that adds a nonlinear chirp to the seed light laser system prior to amplification by the fiber amplifier(s) (e.g., doped fiber or Raman amplifiers). The nonlinear chirp includes an x2 or higher nonlinearity and is configured to compensate for the Self Phase Modulation (SPM) characteristics of the fiber-based amplifiers such that fundamental light is generated that has a spectral E95 bandwidth within five times that of the seed light. When multiple series-connected amplifiers are used, either a single nonlinear chirp element is provided before the amplifier string, or chirp elements are included before each amplifier.

Description

RELATED APPLICATIONS[0001]This application claims priority to U.S. Provisional Patent Application 61 / 670,926, entitled “Reducing The Spectral Bandwidth Of Lasers” filed Jul. 12, 2012.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention generally relates to illuminators used in conjunction with inspection systems, such as semiconductor wafer inspection systems and photomask inspection systems, and more particularly to a fiber amplifier based light source for use with such inspection systems.[0004]2. Description of the Related Art[0005]FIG. 1(A) is a diagram depicting a simplified UV-DUV laser inspection system 40 utilized in the semiconductor industry for inspecting a target sample (e.g., a wafer or photomask / reticle) 41. Inspection system 40 includes an illumination source 50 that generates laser light L50 typically in the UV-DUV range (e.g., a vacuum wavelength less than 350 nm), an optical system 42 including one or more objective lenses 43 that fo...

AI Technical Summary

Benefits of technology

The present invention is about a new way to generate high-quality laser light using a fiber-based system. This system uses a special nonlinear chirp to compensate for the characteristics of the fiber amplifiers used in the system. This compensation allows for the generation of laser light with high peak power and a narrow bandwidth, which is important for high-resolution laser inspection systems. The nonlinear chirp is generated using a single or multiple nonlinear chirp element, and can be added to the seed light or the pump seed light in the amplifier system. The use of a Raman amplifier with both continuous wave and pulsed pump seed light allows for high power levels at wavelengths outside the range of standard fiber lasers. Overall, this invention simplifies the manufacture of high-resolution laser inspection systems by combining the benefits of solid-state lasers with those of fiber-based lasers.

Problems solved by technology

A significant obstacle to the development of short wavelength UV-DUV laser inspection systems is to provide an optical system that can effectively image the UV laser light.

Optical systems manufactured from all fused silica for use in systems using UV-DUV laser light can only handle a limited bandwidth before the performance degrades beyond acceptable limits.

However the increased cost and complexity associated with the use of aspheric surface may not be desirable, and this approach only helps a small amount in most laser systems.

It is also possible to perform the frequency conversion / mixing process using other nonlinear processes (e.g., Raman, parametric generation, and four wave mixing (FWM)), but these techniques can also lead to increased bandwidths and not be suitable for narrow bandwidth optics.

Many stages of frequency conversion / mixing are sometimes needed to generate shorter wavelength light having a specified frequency, and power is lost from the light during each frequency conversation stage.

Solid-state lasers can produce laser light having very narrow bandwidths and high peak power, which allows for the use of less complex (and therefore lower cost) optical systems, but the wavelength choices for solid state lasers are very limited and not suitable for some laser inspection systems, and it can be very challenging to obtain reliable high power light from a solid state laser.

However, as described below, the primary drawback of using fiber lasers to generate high peak power pulsed fundamental light is Self Phase Modulation (SPM).

GHz. This roughly tenfold increase in E95 energy exhibited by fiber-based fundamental light sources is impractical for generating the type of narrow band UV light needed in modern laser inspection sys

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